AD5420 using an external resistor RSET

Q

I want to use AD5420 with current output range of 4mA to 20mA. If internal RSET
is used no problem occurs.
If external RSET is enabled the current is 3,5mA instead of 4mA or 15mA instead
of 20mA. Any idea what I am doing wrong?

A

The issue related to the way in which you are writing to the part, because on
the bench we could re-create the issue on an Eval board and got similar values
to what you are measuring.

So the range must be changed on the write on which the output is enables when
using the REXT range. Please see table 11 of AD5420's datasheet Rev. C.
The following sequence of commands will work and we’ve tried them here.

Reset reg. = 0x0001h to RESET the part after start-up
Control reg. = 0x3005h = 0011 0000 0000 0101b enables the output and external
resistor and sets output range 4mA to 20mA

Alternatively the following will work showing the change of current ranges

Reset reg. = 0x0001h to RESET the part after start-up
Control reg. = 0x0006h = 0000 0000 0000 0110b sets output range 0mA to 20mA
Control reg. = 0x3005h = 0011 0000 0000 0101b enables the output and external
resistor and sets output range 4mA to 20mA